Extrusion apparatus

ABSTRACT

A lightweight plastics extruder is disclosed which can be used with particular benefit in the preparation of structures wherein a foamable plastic is extruded in a desired location and the extruder moved as foam material is deposited.

United States Patent 1191 Wright Oct. 15, 1974 [5 EXTRUSION APPARATUS3,129,459 4/1964 Kullgrecn et al. 425/144 x 3,310,839 3/l967 Son et al425/l50 X [75] memor- Donald Mldland, 3,456,298 1/1967 Foster et al 425149 [73 Assignee; The Dow Chemical Company, 3,526,926 5/1968 Reid425/149 Midland, Mich.

[221 Filed: 1973 Primary Examiner-R. Spencer Annear [21] Appl. No.:342,354 Assistant Examiner-Mark Rosenbaum An ,A r, F -Rb tB.I h RelatedUS. Application Data omey gen or "m 0 er ngra am [62] Division of Ser.No. 96,897, Dec. 10, 1970, Pat. No. 3,764,043, which is a division ofSer. No. 803,348, Feb, 28, 1969, Pat. No. 3,6l9,329. [57] ABSTRACT [52]US. Cl. 425/149, 425/379, 425/817 C [51] Int. Cl B29f 3/06 A lightweightplastics extruder is disclosed which can [58] Field of Search 425/149,150, 141, 162, b d wi h par i ular benefit in the preparation of425/174,8, 379, 4 C, 817 C, 140 structures wherein a foamable plastic isextruded in a desired location and the extruder moved as foam ma-References Cited terial is deposited.

UNITED STATES PATENTS 3,111,707 11/1963 Buckley 425/162 X 2 Claims, 8Drawing Figures EXTRUSION APPARATUS This application is a divisionalapplication of my copending application Ser. No. 96,897, filed Dec. 10,1970, now U.S. Pat. No. 3,764,043 which in turn is a divisionalapplication of my earlier filed application Ser. No. 803,348, filed Feb.28, I969, now U.S. Pat. No. 3,619,329.

This invention relates to an improved extrusion apparatus, and moreparticularly relates to a lightweight extrusion apparatus particularlyadapted for the extrusion of foamable synthetic resinous compositions.

Extruders, particularly of the screw variety, as presently known to theart are relatively massive pieces of machinery which usually aresupported on a fixed base and occasionally are mounted on a mobile basesuch as a truck or trailer body. Extrusion apparatus for handlingthermoplastic resinous materials such as synthetic resins, explosives,rubber and the like often weigh on the order of 2,000 pounds for anextruder of 2 inch screw diameter. Because of the great weight of suchextrusion apparatus, moveable extruders usually are consideredimpractical except in cases where they may be mounted on a railroad car,large truck or trailer body. In many instances, it would be desirable ifthere were available a lightweight extrusion apparatus which was readilyportable, and beneficially a lightweight extrusion apparatus which couldbe used for the preparation of structures such as buildings and thelike. Such lightweight extrusion apparatus would be of particularadvantage in the preparation of synthetic resinous foam products at asite where the foam products were to be used or at a location quite nearthe site where the foam products were to be used. Foamcd syntheticresinous products in general offer a very substantial problem inshipping wherein their great bulk and low weight prescnt a considerableexpense and inconvenience in handling. For example. extruded foamedsynthetic resinous pipe in general is more economically prepared at alocation adjacent its final destination; for example, in providingon-the-site extruded pipe for land drainage and the like. In my earlierU.S. Pats. Nos. 3,206,899 and 3,337,384, there is disclosed a method andapparatus for the manufacture of walled structures from syntheticresinous foam. For the preparation of such structures it is oftendesirable to employ an extruded foam rather than a foam prepared fromexpandable particles or beads. The extruded foam is generally of higherstrength and more suited for most structural applications than foamprepared by foaming and fusing a plurality of particles. It is alsoknown to prepare structures by admixing foam-forming components anddepositing them in a series of layers or turns to provide a structure.Typical apparatus for preparing such materials is shown in U.S. Pats.Nos. 3,336,631 and 3,336,632. Generally. shipping the admixture offoam-forming components permits use of a technique referred to asfoam-in-place wherein the foam is generated at the site where it will beused. Such a procedure can be accomplished either by liquid components.such as a polyurethane foamable composition, or by use of expandableplastic particles, often referred to as foam-in-place beads. Both theliquid foam-forming components and the expandable beads or particleshave some disadvantages: the expandable beads have finite storage life,often mold to a relatively weak structure, and the liquid components cansuffer severely from age ancd contamination from other materials whichimpair their foaming characteristics.

It would be a substantial advantage if there were available an apparatuswhich was readily portable and was capable of extrusion of thermoplasticfoamable resins such as foamed polystyrene, and that the extrusionapparatus be of sufficinetly lightweight construction that it could bereadily handled without the necessity of heavy supports, cranes and thelike.

It would also be beneficial if there were available lightweightextrusion apparatus particularly suited for the extrusion of foamablesynthetic resinous compositions such as thermoplastics and foamablethermosetting compositions which have a thermoplastic or B- stage.

These benefits and other advantages in accordance with the presentinvention are achieved in an extruder, the extruder comprising agenerally hollow cylindrical barrel, the barrel defining a generallycylindrical cavity therein, an extrusion screw disposed within thebarrel and adapted to rotate therein, the barrel having a feed end and adischarge end, means to supply a synthetic resinous extrudablecomposition to the feed end of the barrel, the screw having a feed endand a discharge end, a variable speed drive means associated with thescrew and adapted to rotate the screw in a desired direction to'forwardsynthetic resinous material from the feed end to the discharge end,temperature control means disposed on the barrel between the feed endand the discharge end, the extrusion apparatus being operativelyconnected to a variable power source for the variable speed drive meansand a temperature control means.

Further features and advantages of the present invention will becomemore apparent from the following specification taken in connection withthe drawing wherein:

FIG. 1 schematically depicts a sectional view of a lightweight extruderin accordance with the invention.

FIG. 2 is a schematic representation of an extruder.

in accordance with the invention in association with a power supply andtemperature control means.

FIG. 3 schematically represents an extrusion apparatus in accordancewith the invention adapted to form a structure by the deposition ofsuccessive layers of foamable resinous material.

FIGS. 4 and 5 are views of a forming head which beneficially is employedin cooperation with the extruder of FIG. 3. 4 A

FIG..4A depicts a top view of a portion of the forming head of FIG. 4.

FIG. 6 schematically depicts the controls and power supply employed inthe apparatus of FIG. 3.

FIG. 7 schematically shows a sectional view of a pivot which may beemployed in the apparatus of FIG. 3.

In FIG. 1 there is schematically depicted a sectional view of anextruder in accordance with the present invention generally designatedby the reference numeral 10. The extruder 10 comprises a generallycylindrical barrel 11, the barrel 1] having a feed end 12 and adischarge end 13. The barrel ll defines a generally cylindrical internalcavity 15 extending the entire length thereof. Adjacent the feed end 12,the barrel 11 defines a feed port 16. Affixed to the discharge end 13 isa nose piece 18 having defined therein connecting means 19 adapted toreceive a conduit, or a die to discharge material. An extrusion screw 21is rotatably disposed within the cavity of the barrel 11. The screw 21has an increasing root diameter; that is, decreasing working volume asone progresses from the feed end 12 to the discharge end 13 along thebarrel 11. The screw 21 has an internal cavity 23 extending the lengththereof within which is disposed a conduit 24. The conduit 24 and thecavity 23 extend almost the entire length of the screw and bothterminate in a region remote from the feed end 12 of the barrel 11. Theconduit 24 communicates with the cavity 23. Adjacent the feed end 12 ofthe barrel 11 is disposed a rotary joint 26. The rotary joint 26 definesa first passageway 28 which is in operative communication with thecavity 23 and the screw 21 and a second passageway 29 which is incommunication with the conduit 24. An annular groove 30 disposed aboutthe periphery of the screw 21 provides communication with the passageway28 and a passageway 31 provides communication between the annular groove30 and the cavity 23. A second annular groove 32 is disposed about thescrew 21 and provides communication with a passageway 33 which in turncommunicates with the conduit 24 thereby permitting circulation of heatexchange fluid within the screw 21. Disposed adjacent the annulargrooves 30 and 32 are seals 35, 36 and 37 which beneficially are formedby positioning an O-ring within suitable retaining means defined by thescrew. A motor 39 is operably connected to the screw 21 by means of acoupling 41. The motor 39 is advantageously a hydraulic motor or a highspeed electric gear head motor with a high ratio gear reducer. Betweenthe feed port 16 and the discharge end 13 of the barrel 11 is an outersurface 42 of bare metal. Over the surface 42 is disposed a temperaturecontrol means 43. The temperature control means 43 comprises threezones, a first zone 44 adjacent the feed port 16, a second zone 45adjacent the zone 44 and remote from the feed port. and a third zone 46adjacent the discharge end 13 of the barrel 11. The first zone 44comprises a helically disposed winding 48 of an electrically conductivetubing. beneficially square copper tubing, which is in operativecommunication with a source of heat exchange fluid (not shown).Beneficially, adjacent turns of the winding 48 are in electrical contactwith each other. The heating zone 44 has disposed thereon a winding 49of insulated electrical wire which is in operative communication with asource of electrical power, not shown. A thermally insulating layer 50is disposed over the winding 49. The winding 49 serves as a primarywinding of a transformer and the winding 48 as a single turn shortedsecondary for induction heating of the barrel 1]. The heating zones 45and 46 are of similar construction having windings 48a and 48b.respectively, of hollow electrically conductive tubing in communicationwith heat exchange fluid supply means (not shown) and windings 49a and491; which in turn are enclosed in insulating layers 50a and 50b,respectively. Advantageously, all possible components of the extruder 10are fabricated from light metal such as aluminum, magnesium, titanium,and the operating conditions of the extruder maintained in such afashion that material being extruded therethrough is maintained underpressures of not more than 500 pounds per square inch, and preferablyabout 200 pounds per square inch. Beneficially a barrel liner of nickel,titanium or other corrosion resistant material may be employed ifrequired, as well as a plated screw. Rapid start-up is readily obtainedby passing hot heat exchange fluid through the windings 48, 48a and 48b,as well as within the cavity 23, into the conduit 24 of the screw 21,applying electrical power to the windings 49, 49a, 49b to inductivelyheat the barrel 11. When desired temperature is reached or approached,the supply of heat exchange fluid and power is adjusted accordingly toobtain the desired operating conditions which, of course, are dependenton the material being utilized.

In FIG. 2 there is schematically depicted an extruder in accordance withthe present invention generally designated by the reference numeral 10a.The extruder 10a has a discharge heat control zone 55, an intermediatetemperature control zone 56 and a feed temperature control zone 57. Theextruder 10a discharges into a die 59. The die 59 discharges a foamthermoplastic body 61. A pressure sensing transducer 62 is in operativecombination with the die 59 and senses pressure within the die.Temperature control means 64, 65 and 66 are in operative combinationwith the temperature control zones 55, 56 and 57 which are incommunication with a source of electrical power 68. The screw of theextruder 10a is in operative combination with a hydraulic motor 39a. Thehydraulic motor 39a is in operative combination with a source ofpressurized hydraulic fluid 70 such as a hydraulic pump by means oflines 71 and 72, the pump in turn being driven by a motor 74. Aproportional control valve 76 is disposed within the line 71 and is inoperative connection with a bypass conduit 77 which is also in operativecombination with the line 72. The transducer 62 of the die 59 is inoperative combination with a proportional controller 78 by means oflines 79 and 81. The controller 78 in turn operates the proportionalcontrol valve 76 through lines 83 and 84. The controller 78 in turn isin communication with a power source (not shown) by means of lines 85and 86. As the die pressure increases, hydraulic fluid is bypassedthrough the bypass conduit 77 and the die pressure is maintainedgenerally constant. Operating the extruder 10a at a generally constantdie pressure permits a maximum volume of material to be passedtherethrough at a minimal pressure and permits use of light metal alloysand eliminates high surging pressures within the extruder.

In FIG. 3 there is schematically depicted an extrusion apparatusgenerally designated by the reference numeral 90. The extrusionapparatus comprises in operative combination a pivot or base 91, an arm92 having a first end 93 and a second end 94, the first end 93 beingpivotally affixed to a rotatable support 96. The support 96 in turn issecured to a generally vertically extending stub shaft 97 affixed to thepivot 91. Thus, the second end 94 of the arm 92 is adapted to pivot intwo planes and position the second end on a desired surface to begenerated. As illustrated, the arm 92 can be pivoted within a solidangle. An extruder 10b generally similar to the extruders 10 of FIG. 1and 10a of FIG. 2 is supported on the arm 92 adjacent the second end 94.The extruder 10b discharges into a flexible conduit 99. The conduit 99remote from the extruder 10b terminates in a die 10]. The die 101discharges a heat plastified synthetic resinous foam strip 103 which ispassed to a forming head 105 pivotally affixed to the second end 94 ofthe arm 92 by means of a pivot 106. The foam strip 103 is passed throughthe forming head 105 where it is shaped while still in heat softenedform into a strip 103a and adhered to an adjacent strip 103a.

I the base 91.

FIG. 4 depicts a side view of the forming head 105 of FIG. 3. Theforming head 105 comprises a body 111 upon which are pivotally supportedstructure engaging rolls 113 and 115. A similar pair of structureengaging rolls 114 and 116 are oppositely disposed. The structureengaging rolls 113, 114, 115 and 116 are driven by means of a powersource (not shown) within the body 111 such as a hydraulic motor. Thedrive arrangement of the forming head 105 is generally similar inconstruction to that shown in my earilier U.S. Pat. No. 3,337,384. Alsopivotally supported by the body 111 are transverse foam engaging rolls118 and 119 adapted to engage a freshly extruded strip of foam 103. Apair of side foam engaging rolls 121 and 122 are rotatably supported bythe housing 111. The foam shaping rolls 118, 119, 121 and 122 arefriction driven by the strip 103. The rolls 118, 119, 121 and 122 definea generally rectangular channel therebetween and force the heat softenedstrip 103 into a generally rectangular cross-sectional configuration.Also pivotally supported by the body 111 are first and second sealingplates 124 and 125 which extend transversely and gen.- erally parallelto the rolls 118 and 119. The plates 124 and 125 act also as supportmembers for the forming head 105 when in engagement with a structurebeing generated. It is advantageous to employ two plates in that thehead will prepare smaller diameter structures than if only a singlesealing plate is employed. The sealing plates are internally heated andadapted to raise the temperature of the adjacent surface of the strip103 to a heat sealing temperature and the surface of an adjacent strip103a also to a heat sealing temperature. A resiliently loaded followerroll or pressure means 131 is supported by the body 111 and adapted toforce the heated surface of the strip 103 into engagement with theheated surface of the strip 103a. On cooling, the strip 103 is heatsealed to the strip 1030. A strip sensing means and control element 126is affixed to the body 1 1] remote from the follower roll assembly orpressure means 131. The control element 126 has strip engaging members127 disposed on opposite edges of the strip and forming free opposedcorners of a rectangular four-bar linkage 128. The corners of thefour-bar linkage supporting the members 127 are resiliently tensionedtoward each other by means not shown. One of the remaining pivot pointsof the four-bar linkage is pivotally affixed to the support or base 111while the opposite pivot is in operative engagement with a linearactuator 129 of a control means 130. Thus. as the fourbarlinkage 128 isfree to flex at any of its corners and move laterally. the signaltransmitted to the controller 130 is proportional to the width of thestrip only and not to the precise lateral location of the strip.

FIG. 4A depicts a top view of the strip sensing means and controlelement 126 in engagement with the strip 103. As the strip becomeswider, the head is driven about the periphery of the structure at a morerapid rate to maintain the desired width of the strip by stretching ofthe incompletely cooled foam. If the strip becomes narrow, the head 105is driven more slowly and the width of the foam strip increases.

In FIG. 5 there is depicted a view of the forming head 105 with thefollower roll 131 removed to more clearly show the relative positions ofthe structure engaging rolls and 116, the shaping rolls 118, 119, 121and 122.

FIG. 6 schematically depicts the control arrangement for an apparatussuch as the apparatus 90 of FIG. 3 omitting the heat exchange fluidconnections and controls. An extruder 10c and the forming head 105 areoperatively connected through the pivot or base 91 which provides arotary joint which carries the supply requirements of the extruder tothe forming head 105. The power source 108 comprises a source ofcompressed air 140, a source of alternating electrical current 141, afirst hydraulic fluid supply or pump 142 and a second hydraulic fluidsupply or pump 143. The first hydraulic pump is operatively connected toa hydraulic motor 39b by means of lines 144 and 145. A bypass valve 760is disposed within the line 144 in a manner similar to the valve 76 ofFIG. 2. The valve 76a is controlled by a transducer 62a which sensesinternal pressure of the die. The transducer operates a proportionalcontroller 78a which in turn is in operative communication with thepower source 108. The controller 78a and bypass valve 76a are operatedto maintain a generally constant die pressure within the die 101. Asource of synthetic resin 146 is in operative combination with the airsource 140. Air from the source entrains granular material from thesource 146 and delivers it to a feed hopper 147 in operativecommunication with the extruder 100. The alternating current source 141is in operative communication with the proportional controller 78a andtemperature control zones 149, 149a, 14% and 149C through supply lines151 and 152 and power or temperature control units 153, 153a, 1531) and153C, respectively. For the sake of clarity, lines depicting heatexchange fluid supply to the extruder 100 have been omitted. The secondsource of hydraulic fluid 143 is in operative communication with ahydraulic motor 154 in operative communication with the forming head 105through lines 157 and 158. The strip sensing and speed controlarrangement 126 for the motor 154 is depicted in FIGS. 4 and 4A.

In FIG. 7 there is schematically depicted a sectional view of a pivot orbase 91a such as is employed in the apparatus of FIG. 3. The base 91acomprises a fixed support having rigidly secured thereto a connector andconduit block 166 having a first pair of passages 167 in communicationwith passages (not shown) within the block 166 and forming the inlet andoutlet of a first hydraulic fluid circuit. A pair of passages 168 arealso in communication with passages (not shown) within the block 166 toform the inlet and outlet of a second hydraulic circuit. The block 166defines an opening 169 adapted to receive a fluid cooling media such asair and also in communication with a passage within the block 166 (notshown). A pair of openings 170 are also disposed within the block 166and are in communication withelectrical conduit passages (not shown).Rigidly afflxed to the block 166 is a stub shaft or mandrel 171 havingdefined therein first and second passages 167a which are in operativecommunication with the openings 167 of the block 166 and terminate inannular recesses 167b circumferentially disposed about the shaft ormandrel 171. The openings 168 are in communication with passages 168awhich in turn terminate in annular recesses 16812 similarly disposed tothe recesses 16717. The opening 169 is in communication with a passage169a which terminates in an annular passage 16912. The openings 170 arein operative communication by means of insulated electricalconductorstnot shown) with a plurality of first slip rings 170a in powercircuits for heaters and the like and a plurality of slip rings 17%which are employed for control circuit purposes. A rotary sleeve 174 isdisposed over the mandrel 171 and is adapted to pivotally rotatethereabout. The sleeve 174 has a pivot 96a which pivotally supports aboom or elongate member 930. The sleeve 174 defines passages andconnection means 1676' which are in operative communication with theannular grooves 167!) as the sleeve 174 is rotated about the mandrel171. Similarly, the openings 168 are in communication with the passagesand connection means 1680 and the opening 169 is in operativecommunication with a connecting means and passages l69c. Suitablesealing means (not shown) such as rings are disposed adjacent theannular passages 167b, 168!) and 16912 to provide a fluid tight seal oneither side of each of the annular recesses. Suitable electricalconnections 170 are provided to the slip rings 170a and 17012. Thesleeve 174 supports a first brush and connector assembly 170(- inoperative communication with the slip'rings 170a and a brush andconnector assembly 170d in communication with the slip rings 170b.Suitable electrical and hydraulic fluid cooling connections are madefrom the openings 167c, 168c and 169C and the connector brush assemblies170(' and 170a to operating elements such as the extruder forming headand the like supported by the elongate member 930, the connecting linesbeing of sufficient length to permit rotation of the boom about the axisof the pivot 96a disposed at right angles to the axis ofthe mandrel 171permitting the boom with its associated fluid conduits and electricalcables to traverse at least a major portion of the solid angle withoutthe necessity of rotation of supply lines in communication with thevarious openings in the block 166.

In operation of apparatus in accordance with the present invention forthe preparation of a building structure, apparatus generally inaccordance with FIG. 3 is employed. A tapering starter strip is extrudedfrom the die 101 generally as disclosed in my previous U.S. Pats. Nos.3,206,899 and 3,337,384 and successive turns of a helix deposited in apredetermined pattern until the desired height of structure is obtained.Beneficially, employing a remote power source, hydraulic or electricmotors are advantageously utilized to drive the screw of the extruderand to operate the forming head 105. By employing the controlarrangements such as are set forth in FIGS. 2 and 6, the rate ofextrusion of the extruder as well as extrusion conditions can be closelymaintained. By employing a pressure transducer such as the transducer 62to activate a proportional controller such as the controller 78, thescrew speed is varied in such a manner that a generally constant diepressure is obtained and the pressure rating of the extruder barrel isnot exceeded, assuming, of course, that adequate heat input ismaintained to the heating zones of the extruder. Beneficially, thetemperature of the various zones in the extruder and the screwtemperature are remotely controlled from the power source. Whenelectrical heating is utilized, suitable silicon controlled rectifiercircuitry well known in the art is employed with particular benefit.

ln a similar manner, the sealing plates or shoes 124 and 125 areadvantageously electrically heated and the temperature thereofcontrolled by varying the power supplied thereto such as by the use ofsilicon controlled rectifiers or other conventional power control means.A forming head such as the head is driven by an electric or hydraulicmotor. The speed of the head is controlled by the sensing and controlelement 126. By virtue of motion imparted by the four-bar linkage 128,the linear actuator 129 conveniently may operate a potentiometer,variable resistor, hydraulic valve, pneumatic pilot valve or otherconventional signal generating element for control of the forward speedof the head 105. Thus, minor variations which occur in the extruder andare not controlled by either temperature or flow rate are corrected bythe depositing head 105.

Beneficially, a wide variety of foamable materials are extrudable fromsuch an apparatus including thermoplastics and extrudable thermosetswhich cure at a rate to permit the extrusion and heat sealing ofadjacent strips to each other. A foamable material may be fed to thefeed hopper of the extruder; e.g., foamable polystyrene granules; thatis, polystyrene having a suitable liquid or solid blowing agent withinor on the particles. Alternately, a non-foamable material may be fed tothe extruder and the blowing agent incorporated therein by pumpingdirectly into the barrel at a location where the material is in heatplastified form and the blowing agent admixed therewith by action of theextrusion screw. Most often, it is desired to feed the extrudablematerial from a location remote from the extruder. This is readilyaccomplished by pneumatic conveying through the base pivot to theextruder hopper at the extruder, or alternately, the hopper may befilled periodically from an external source as the apparatus rotates.Such materials are well known in the art and need not be furtherdiscussed. Usually it is desirable to employ foam for construction;however, non-foamable extrudable materials may also be employed ifdesired.

The extrusion die such as the die 101 of FIGS. 3 and 6 or the die 59. ofFIG. 2 may be of the conventional foam forming variety wherein thefoamable heat plastified material is discharged from a relatively smallconfiguration and permitted to expand in a nozzle, bazooka or otherrestraining and shaping means which serves to form the foam to a desiredconfiguration; for example, in the practice of the present invention,the foam is formed to a sectionhaving a cross-sectional configurationapproximating a rectangle. Alter'nately and advantageously, the foamableheat plastified material may be extruded from a die having theconfiguration of a pair of crossed slots or an X-configuration. Thefoamed extrude issuing from such a slot approximates a square which isfurther shaped into the desired configuration by rolls such as the rolls118, 119, 121 and 122. Alternately, a round, rectangular or otherdesired shape tube is employed with benefit.

Generally for most structural purposes, it is desirable to employ afoaming mixture which does not utilize a plasticizing-type blowingagent; that is, a blowing agent having a high solubility in the polymer,but a blowing agent which has limited or low permeability through thepolymer forming the cell walls of the resultant foam to assure minimumdeformation of the foam on aging. Usually, it is desired to prepare arigid structure having dimensional stability. However, minor shrinkageand warping can be tolerated, particularly if metallic reinforcing isincorporated in the joint between adjacent strips in the manner setforth in my earlier U.S. Pats. Nos. 3,206,899 and 3,337,384.

By way of further illustration, an extrusion apparatus generally inaccordance with the arrangement shown in FIG 1 is constructed employinga magnesium alloy extrusion screw having a diameter of about 2 inchesemploying steel tube as a barrel and one-quarter inch square coppertubing with 0.040 inch wall thickness as the induction heating and heattransfer elements corresponding to the elements 48, 48a and 48b ofFIG. 1. An electrical resistance winding is disposed over and insulatedfrom the winding of copper tubing. Each winding is about 1, l 50 turnsof number 17 copper wire and with an applied voltage of 220 volts ACdraws a current of 20 about 7.3 amperes. The control system is generallyas shown in FlGS. 2 and 6 and a hydraulic motor coupled to the screw.Polystyrene containing about 6 percent dichlorodifluoromethane isextruded at a rate of about 200 pounds per hour to provide asatisfactory foam. The extruder, including hydraulic motor, feed hopper,pressure transducer, weighs about 125 pounds. The extruder issubsequently fixed to a boom and forming head such is shown in FIGS. 3,4 and 4A and a generally hemispherical structure of foamed polystyreneabout 3 inches thick, 40 feet in diameter and 20 feet high is depositedin a period of about 16 hours. A four foot diameter hole in the topcenter is subsequently filled by hand with a foam cap. Granularpolystyrene particles containing about 5 weight percentdichlorodifluoromethane are conveyed by means of an air conveyor to thefeed hopper.

In a manner similar to the foregoing illustration, foamable plasticcompositions are readily extruded employing a lightweight extruder andsuch an extruder is particularly suitable for the preparation of walledstructures.

As is apparent from the foregoing specification, the

present invention is susceptible of being embodied with variousalterations and modifications which may differ particularly from thosethat have been described in the preceding specification and description.For this reason, it is to be fully understood that all of the foregoingis intended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention.

What is claimed is:

1. A screw extruder particularly adapted and suited for deposition ofmaterial on a fixed base or support, the extruder comprising incooperative combination a barrel having a feed end and a discharge end,the barrel defining a generally cylindrical cavity, the barrel having aplurality of heating zones, each of the zones having means toinductively heat the zone and fluid heat transfer means associatedtherewith,

an extrusion screw disposed within said cavity,

means to rotate said screw at a variable rate,

a pressure sensing means disposed adjacent said discharge end to sensethe pressure of extruded material discharging from said cavity,

a control means in operative association with said pressure sensingmeans and said means to rotate the screw to thereby maintain a generallyconstant output pressure,

a forming head positioned to receive an extrude from I the extruder andform the extrude to a desired configuration, the forming head having inoperative combination therewith means to detect a cross-sectionaldeviation of the extrude in operative combination with means to propelsaid head and extruder, to thereby maintain a generally constantcross-section of said extrude. 2. The extruder of claim 1 wherein thebarrel has a copper jacket defining heat transfer fluid passages andinductive heating means disposed about the copper

1. A screw extruder particularly adapted and suited for deposition ofmaterial on a fixed base or support, the extruder comprising incooperative combination a barrel having a feed end and a discharge end,the barrel defining a generally cylindrical cavity, the barrel having aplurality of heating zones, each of the zones having means toinductively heat the zone and fluid heat transfer means associatedtherewith, an extrusion screw disposed within said cavity, means torotate said screw at a variable rate, a pressure sensing means disposedadjacent said discharge end to sense the pressure of extruded materialdischarging from said cavity, a control means in operative associationwith said pressure sensing means and said means to rotate the screw tothereby maintain a generally constant output pressure, a forming headpositioned to receive an extrude from the extruder and form the extrudeto a desired configuration, the forming head having in operativecombination therewith means to detect a cross-sectional deviation of theextrude in operative combination with means to propel said head andextruder, to thereby maintain a generally constant cross-section of saidextrude.
 2. The extruder of claim 1 wherein the barrel has a copperjacket defining heat transfer fluid passages and inductive heating meansdisposed about the copper jacket.